The invention relates generally to spark-erosion machines, and more particularly to a method and apparatus for tensioning a wire shaped metal working electrode in a spark-erosion machine.
As used herein, the word xe2x80x9cwire tensioningxe2x80x9d relates principally to the drawing force exerted on the metal working electrode under the assumption of a constant electrode diameter.
An example of a wire tensioning system and procedure in accord with the generic type is disclosed by DE 196 07 705 A1. In this instrument the wire shaped metal working electrode (hereinafter xe2x80x9celectrode wirexe2x80x9d) is withdrawn from a supply reel and guided over several directional change rollers to a braking roller. The electrode wire circumferentially embraces the braking roller, then runs over further directional change rollers to a clamping roller-pair, and from this to a container for waste recycling. For the automatic startup threading of the electrode wire about the braking roller, DE 196 07 705 A1 further proposes to furnish injector nozzles to produce jets of a through-flowing fluid. This arrangement is so constructed, that during the introduction of the electrode wire, a fluid flow tangential to the braking roller is produced, so that the electrode wire is guided about the circumference of the braking roller.
The establishment of the wire tensioning is done with the aid of three motors, which drive the supply spool, the braking roller, and the clamping roller-pair. In this combination of drivers, the clamping roller-pair motor determines the desired transport speed of the electrode wire. The braking roller generates, with the aid of the thereby caused friction effects, the specified electrode wire tensioning especially in the working area within the confines of the workpiece to be machined. When the electrode wire is brought to the electrically conductive workpiece, there occurs an electrical discharge, and a material removal occurs in accord with the known technology of spark erosion machining. By means of a relative motion between the electrode wire and the workpiece, the desired shaping can be achieved. In any case, where the spark erosion process is concerned, forces are generated, for instance from electromagnetic and electrostatic fields, which lead to a deviation in path of the electrode wire. In order to reduce this deviation, the wire electrode is tensioned, as has been explained above.
It is desirable to make available an improved wire tensioning system and a better procedure for tensioning of an electrode wire than can be supplied by the present conventional systems as described above.
In accord then, with this purpose, a wire tensioning system is disclosed for an electrode wire, or the like, of a spark erosion machine, in which the electrode wire circumferentially and frictionally embraces a braking roller placed in the wire entry zone and/or also so embraces a tensioning roller placed in the wire withdrawal zone. Further, the braking and/or the tensioning roller is provided with attendant fluid through-flow nozzles which are so designed that they engender in the wire entry zone and/or the wire withdrawal zone a pulling force on the electrode wire for the generation of a basic tension.
The procedure, then, is for the tensioning of an electrode wire, or the like, in a spark erosion machine, wherein the electrode wire, at least partially frictionally and circumferentially embraces a braking roller in the wire entry zone and/or so embraces a tensioning roller in the wire withdrawal zone. Additionally, an electrode wire is further placed under tension by means of one of the fluid through-flow nozzles associated with the braking roller and/or the tensioning roller, for the generation of a basic tension in the electrode wire entry zone, and preferably also in the electrode wire withdrawal zone.
In this procedure, under the concept of xe2x80x9cbraking rollerxe2x80x9d, an optional roller is placed in the electrode wire entry zone. That is, as one looks in the wire travel direction, the braking roller is located in front of the operational position of the spark erosion machine (i.e., the position in which the workpiece is situated). Preferably, however, the roller is proximal to an upper electrode wire guide-head of a cutting erosion machine. Conversely, the xe2x80x9ctensioning rollerxe2x80x9d, is to be found (again looked at in the direction of the electrode wire travel), following the operational position of the spark erosion machine, that is, in the electrode wire withdrawal zone, preferentially proximal to the lower electrode wire guidance head of a cutting spark erosion machine.
The fluid through-flow nozzles associated with the braking and the tensioning rollers, fulfill the purpose of generating a basic tension of the electrode wire in both the wire entry and wire withdrawal zones, and do so before the braking roller and following the tensioning roller, as seen in the travel direction of the electrode wire. This is carried out in that the nozzle jet engenders a tensile force counter to that of the braking or tensioning roller on the already introduced electrode wire. In this way, the electrode wire comes into a frictionally conditioned effective engagement to grip the braking and/or the tensioning roller. The effective tensioning in the operational position of the spark erosion machine, in which position the workpiece lies (that is, the tension, as seen in the travel direction of the electrode wire, immediately behind the brake roller and directly in front of the tensioning roller), is built up in accord with the cable-friction principle under which the braking and/or the tensioning roller operates.
In this way, in a particularly advantageous manner, the effect can be of value. The value lies in the fact that already a relatively small tensile force, or tensile force change, effected by the nozzles, is sufficient to attain the necessary effective tension. In other words, this means essentially exerting influence on the effective tension. The electrode wire embraces, namely, the braking and/or the tensioning roller. Because of the basic tension generated by the accelerating/decelerating action of the nozzle jets, the wire remains in an effective grasping contact with the respective roller, because of frictional forces between the electrode wire surface and the roller periphery. In accord with the cable friction formula in accord with Euler, the following equation is valid:
F2=F1xc3x97excex1xcexc
where
F1 is the basic tensile force generating the basic tension;
F2 is the effective tensile force generating the effective tension;
xcex1 is the angle of wrap around the circumference; and
xcexc is the coefficient of friction.
In accord with this formula, the effective tensile force is essentially proportional to the basic tensile force. If the product of the wrap angle and the friction coefficient is large enough, then the system is self restraining (i.e., F2 becomes independent of F1 where F1=0).
In accord with the above, the following can be attained, among other advantages:
greater tensile force in the operational position,
higher reliability
better operator friendliness
better possibilities for automation of the electrode management
It is advantageous, if the nozzle is designed as a venturi nozzle, bringing about a flow of fluid, and thereby making use of the tensile force on the electrode wire. The fluid especially preferred is that operating fluid which is itself designed for spark erosion. Preferably, the nozzle is an injector nozzle. This can, for instance, be constructed as a two-chamber nozzle with two, chambers placed essentially coaxially to one another.
Advantageously, the electrode wire is penetratively run through one of the chambers of the injector nozzle. The outer nozzle chamber, is preferably connected to a pressure fluid supply. Particularly advantageous is a situation in which the inner nozzle chamber structurexe2x80x94as seen in the travel direction of the electrode wirexe2x80x94is extended to protrude beyond the outer nozzle chamber. Also, it is advantageous, if between the nozzle and the braking and/or tensioning roller to which the nozzle is assigned, no other additional rollers, such as direction-change rollers or the like, are to be found. The nozzle should be further in the general proximity of the roller to which it is assigned.
Particularly of value, beyond the above, a means is provided, which applies to the tensioning roller a moment of rotation and/or an additional means for applying a moment of rotation to the braking roller. For this purpose, for instance, corresponding motors can serve, which, respectively, drive or brake the rollers. For instance, the means can be a tensioning roller motor, which drives the tensioning roller, and the additional means a braking roller motor which brakes the braking roller.
The nozzle which evokes the frictional action is preferably so designed, that the tensile force exerted by it upon the electrode wire is directed away from the associated roller. For example, the nozzlexe2x80x94as seen in the travel direction of the electrode wirexe2x80x94can be located behind the tensioning roller. It can further be worthy of consideration, to place the nozzle before the braking roller, that is to say, in front of the working station. Particularly advantageous is a design, wherein the nozzle is (seen in the travel direction of the wire), located behind the braking roller in the wire withdrawal zone, i.e. behind the operational position with the workpiece. The nozzle is, in this case, located relatively distant from its associated braking roller. Of advantage, the tensioning roller can be totally dispensed with. This design of the electrode wire tensioning system is of particular value in the case of relatively fine electrode wires. In this case, the jet produced by the nozzle generates the effective tensile force. The basic tensile force in front of the brake roller must be built up by an additional nozzle or by means of another auxiliary device such as weight tensioning or an idler roller.
An electrode wire tension system which exhibits a tensioning roller and wherein the nozzle is one of the nozzles associated with the tensioning roller, which is located behind (as seen in electrode wire travel direction), the tensioning roller is particularly advantageous. This nozzle further exerts force on the electrode wire in a direction away from the tensioning roller. In this design, for instance, the braking roller can be dispensed with. It is particularly advantageous, nevertheless, if additionally a braking roller is provided, especially a braking roller of the above mentioned design, that is, with a tension producing nozzle assigned thereto. In this case, the basic tensile force is built up by means of the roller, that is, by the drive means assigned thereto.
It is of advantage, if the electrode wire at least nearly circumferentially wraps around the braking roller and/or the tensioning roller and finds itself in effective, gripping, operational contact with the braking roller and/or the tensioning roller because of frictional forces. The wrap-around angle runs generally less than 360xc2x0, particularly between 310xc2x0 and 350xc2x0, and most preferred is 330xc2x0. The wrap-around angle can alternately also be greater than 360xc2x0. For instance, the electrode wire can make multiple wrap-arounds about the corresponding roller. By that means, with the intervention of fluid flow through the nozzles a substantial frictional action on the rollers is achieved, and therewith an exceptional attainment of force transmission to the tension of the electrode wire occurs, without damage to the electrode wire or a wire overlapping on the rollers.
In a further preferred example, in the case of the wire tensioning system, additional means are provided for the measurement of the tension on the electrode wire. Advantageously, the electrode wire is guided to one or two directional change rollers. When this is done, at least one of the two directional change rollers is made elastically resilient by means of a spring anchorage. The measurement means determines the position of these directional change rollers, the spring means, and/or further auxiliary means of the tension of the respectively present electrode wire. The data so obtained makes it possible, that a control device can regulate the fluid flow produced from the nozzles in relation to the wire tension measured and/or in relation to the set point values.
In this way, the tensile force activated by the nozzle and applied to the electrode wire, and therewith also its basic tensile force, can be influenced. Thus, the effective tensile force available at the work position, that is the effective wire tension, can be controlled. Variations in the basic tensile force and therewith in the resulting effective tensile force have a negative effect on the quality of metal working.
Principally, the tensioning roller and/or the braking roller are preferably constructed as cylindrical disks, whereby, advantageously, symmetrically within the rim surfaces, a uniform, circumferential groove is provided, and the groove narrows itself toward the center of the side surfaces and in the direction of the disk axis. Such rollers make possible a practically vibration-free precise electrode wire guidance and thus improve metal working quality on a workpiece. The rollers are also capable of handling wires of varying diameters. For example, by means of a V-shaped groove, the frictional force between the braking rollers (as well as the tensioning roller) and the electrode wire is increased.
Advantageously, in the case of the braking roller and/or the tensioning roller a spiral threaded guide to and/or a threaded guide return for the electrode wire is provided to maintain the electrode wire in the desired center groove. For instance, upon an initial introduction of the electrode wire, this wire could find itself displaced from the center axis (i.e., from the xe2x80x9cV-notchxe2x80x9d) of the corresponding roller. With the aid of the guide-in and guide-back means, the electrode wire is guided to (or back to) the central circumferential groove. Also the electrode wire, in case it jumps out of the above mentioned groove during operation of the machine, is transported back to its proper place. Threaded windings of this sort can be optionally installed not only in the brake and tensioning rollers, but also in optional other rollers of the spark erosion machine. Advantageously, for such a roller, two threadings are provided, which allow for wire returning in opposite axial directions.
Beyond this, such a roller is advantageously built in two parts, whereby from two parts one interposed groove is formed and is used for the conducting of the electrode wire.
Finally, in an advantageous example, in a fluid flow channel, in which the electrode wire is transported, that is to say, is carried along, one or more relief borings are provided, through which energy-poor fluid can be released. These relief borings are advantageously located at such positions where a substantial part of the next available kinetic energy has already been transferred to the fluid, that is, near to a flow accelerating nozzle. This prevents a situation, in which slow moving, and hence energy poor fluid interferes too strongly with newly incoming, energy rich fluid. Further, a sequential, continual increase in volume flow is avoided.